[@Veridis Quo]Hmm...there are a lot of things you can involve a sacrifice with. You may put it this way: Upon transforming into a demonic creature, Luke's strength and power are amplified to a certain extent. Through this, he can create a bond with an ally by blood ritual. (or whatever you call the 'getting one's blood in exchange of supernatural powers') With that bond formed, the ally is able to share a sum of Luke's strength while at his demon form. The ally is able to have augmented physical prowess and agility BUT he/she is only limited to partake it at a very short amount of time. It's like power gifting but like others, there should be some obvious drawbacks. 

Also, there isn't a need to add more psychological affliction to your character. His current age just needs to be on the appropriate scale, one which he attained mental stability despite having such powers. 


EDIT: I guess I've misread this part.

[quote]"So I'm alright with scrapping the demon summoning but I want him to have something else to use when he's not using his trump card."[/quote]

This can go to a lot of ways. He can still use his power gifting while in human form but the strength and agility granted isn't as potent as when he's at metamorphosis but it's still strong nonetheless. Just tell me your thoughts about these. If it still doesn't fit right, or you want something else, I can provide more possible ideas.